When a glass containing an alkali component is used as a glass substrate constituting a thin film transistor drive color liquid crystal display (TFT-LCD), an alkali ion in the glass substrate is eluted to deteriorate TFT properties, or the glass substrate may be broken during heat treatment since the thermal expansion coefficient of the glass increases. As a glass substrate for TFT-LCD, therefore, it is general practice to use an alkali-free glass containing no alkali component (for example, see JP10-59741A).
Meanwhile, in recent years, displays including a liquid crystal display are steadily growing in size, and in glass substrates for use in the displays, for example, it has come to be accordingly required to decrease the amount of residual bubbles in the glass substrates or decrease the weight of the glass substrates.
In the production process of a glass substrate, removing bubbles, etc. in a glass substrate is referred to as refining treatment, and the refining treatment is generally carried out by adding a refining agent to a glass melt. In particular, arsenic oxide, antimony oxide, etc., are suitably used as a refining agent for glass substrates for liquid crystal displays. When a glass reaches a high temperature from a low temperature, these refining agents cause a reaction of MOx→MOy+zO2↑ involving a change in the valence of a metal constituting the refining agent, and oxygen generated in the above reaction enlarges bubbles that are caught in a glass melt and causes the bubbles to float upward to remove them.
Since, however, the above arsenic oxide, antimony oxide, etc., which are known to have high refining effects, are matters of concern regarding environmental damages, it is socially called for to decrease their use and discharge amounts.
Therefore, JP10-59741A has come to report a method of removing bubbles by incorporating 0.05 to 2% of SnO2 into an alkali-free aluminoborosilicate glass without using any arsenic oxide.